The invention concerns a terminal and antenna system for transmitting and receiving data to and from non-geostationary satellites in low Earth orbit.
The terminal and antenna system is designed to be inserted into a system for transmitting data at high bit rates to and from a constellation of satellites for public or private, civil or military use.
A constellation of this kind comprises a large number of non-geostationary satellites in low or medium orbit around the Earth. In a standard configuration, the altitude is in the range from 800 km to 1500 km and the satellites are regularly spaced in a series of orbital planes, with eight satellites at 45.degree. from each other, for example, in each of eight steeply inclined orbital planes around the Earth, so that any point on the globe is at all times in view of at least two if not three satellites. The choice of a low orbit for the satellites is motivated by the requirement for a high level of interactivity with a station centralizing access to networks, at high data bit rates and therefore with receivers that receive a high power level, which is not compatible with the propagation time via the geostationary orbit. However, this choice leads to fast movement of the satellites across the sky, a satellite in a 1500 km orbit remaining in view from a point on the ground for about 10 minutes only.
To reduce the number of satellites assuring continuity of calls with terminals on the ground, it is necessary for the terminals to be able to track the satellite for as long as possible, and therefore as far as possible towards the horizon. A second condition for these terminals is that they must be able to switch the stream of calls very quickly from a satellite reaching the horizon to a more visible satellite. Finally, the gain of the antenna must be in the order of 30 dBi for the transmit and receive beams.
Solutions to this problem have been proposed. A first solution uses an electronically scanned antenna, but the angular range to be covered is very wide (0.degree. to 360.degree. in azimuth, 10.degree. to 90.degree. in elevation), in which case this solution implies a prohibitive number of active elements: phase-shifters, low-noise receive amplifiers and transmit power amplifiers between the radiating elements and the phase-shifters to compensate for their losses and those of the splitters/combiners. Their cost is therefore much too high.
Another solution, originating in the military field, and for tracking a plurality of moving, over-the-horizon targets is disclosed in U.S. Pat. No. 3,755,815 and described in Microwave Journal (Oct. 75, pp. 31-34). It uses an array of active transmitter elements associated with a dome lens of dielectric material for deflecting the beam to the horizon and beyond. That solution has the major drawback of very high manufacturing cost because it requires an array of several hundred active elements.
There are other means for deflecting radio beams by using dielectric or waveguide lenses, for example as described by Lo and Lee in "Antenna Handbook", but their technology restricts them to small deflection angles, of approximately 10.degree. about the axis of the lens, and with no target tracking capability.
In the field of microwave antennas, the literature (see for example PCT WO 88/09066) describes antennas incorporating a plane array antenna associated with a focusing microwave lens and a horn source that can be positioned on a portion of a focal sphere depending on the direction required of the beam. Those antennas have the drawback that the radiating surface is a plane array so the directivity of the antenna falls off drastically at low elevations (approximately -7.6 dB for an elevation of 10.degree.), whereas here the requirement is for constant directivity.